When a human skeletal joint is damaged, whether as a result of an accident or illness, a prosthetic replacement of the damaged joint may be necessary to relieve pain and to restore normal use to the joint. Typically the entire joint is replaced by means of a surgical procedure that involves removal of the ends of the corresponding damaged bones and replacement of these ends with prosthetic implants. This replacement of a native joint with a prosthetic joint is referred to as a primary total-joint arthroplasty.
For a damaged human knee, the total knee is commonly replaced with prosthetic components shaped to replace portions of the distal femur, proximal tibia and patella. Prosthetic components for use in replacing the distal femur are shaped to replace the articulating surfaces of the medial condyle, lateral condyle and trochlea, and prosthetic components for use in replacing the proximal tibia are shaped to replace the tibial plateau. Commonly, the tibial component is two-piece: a tibial tray is affixed to the bone and a bearing is received on the tibial tray. The tray is commonly made of metal, such as a cobalt-chrome alloy or titanium alloy, and the bearing is commonly made of a polymer material such as ultrahigh molecular weight polyethylene.
Several alternative designs for total knee prostheses have been available in the past. Some knee prostheses are designed for use when the cruciate ligaments are retained; some are designed for use when the cruciate ligaments are removed. Some of these alternative knee prostheses can be distinguished by the type of tibial components used: some designs provide a fixed bearing, wherein the position of the bearing component is locked with respect to the tibial tray; other designs allow for movement of the bearing with respect to the tibial tray. The designs that allow for movement of the bearing component with respect to the tibial tray are commonly referred to as mobile bearing designs or rotating platform designs.
In fixed bearing designs, the tibial trays commonly have sidewalls, projections, flanges or dovetails that mate with features on the bearing component to lock the tibial tray and bearing component together. In contrast, in rotating platform or mobile bearing designs, the superior or proximal surface of the tibial tray typically does not include such sidewalls or surface features; instead, the proximal surface of the tray is usually planar and smooth, to enable the bearing insert to rotate. In typical fixed bearing designs where the bearing and the base are locked around their peripheries, the size of the bearing component is dictated by the size and features of the tibial base component, so that the bearing component is properly fixed in position on the base; accordingly, the match between the femoral component and the bearing component may not be ideal. In mobile bearing designs, since the bearing component does not need to mate with any locking mechanism around the periphery of the bearing or base, the surgeon has greater flexibility in ensuring an ideal match between the bearing component and the femoral component.
The decision as to whether to use a fixed bearing design or mobile bearing design is based on a number of factors, including surgeon preference and the needs of the individual patient. For example, use of a mobile bearing design may provide the patient with a more natural feel to the joint. However, if the soft tissue of the knee joint is in poor condition (e.g. the collateral ligaments are compromised), it may be advisable to use a fixed bearing design to provide the patient with greater stability. Sometimes, the decision as to whether to use a fixed or mobile bearing design is not made until the condition of the patient's soft tissue can be evaluated intraoperatively.
On occasion, the primary prosthesis fails. Failure can result from many causes, including wear, aseptic loosening, osteolysis, ligamentous instability, arthrofibrosis and patellofemoral complications. When the failure is debilitating, revision surgery may be necessary. In a revision, the primary prosthesis is typically removed and replaced with components of a revision prosthetic system.
In the case of a primary knee prosthesis utilizing a mobile bearing design, the revision procedure may involve changing the prosthesis to a fixed bearing design. This change can result from several factors, such as deterioration of the knee ligaments. Because the features of the tibial base components of fixed and mobile bearing designs are vastly different, revision from a mobile bearing design to a fixed bearing design with existing implant systems typically requires removal of the tibial base component and replacement with a tibial base component that is appropriate for a fixed bearing configuration.
Accordingly, there is a need for a prosthetic knee system that allows the surgeon to choose intraoperatively whether to use a fixed bearing or mobile bearing implant system. In addition, there is a need for such a system that allows the surgeon to focus on providing the optimum match between the tibial insert and the femoral component in a fixed bearing option, rather than being required to select a particular tibial insert bearing size to match the tibial tray. Moreover, there is a need for a prosthetic knee system that allows for a change from a mobile bearing design to a fixed bearing design without removing the tibial base component from the tibia. Conversely, there is a need for a system that allows for conversion from a fixed to a mobile bearing design. This may occur if a surgeon observes a great deal of joint space narrowing on an x-ray or during a revision surgery. In this situation, the surgeon may decide to replace the fixed bearing insert with a lower wearing mobile bearing insert.
Prior art systems providing designs for forming either a fixed or a mobile bearing include U.S. Pat. No. 6,709,462 entitled “Modular Joint Prosthesis System,” and U.S. Pat. Publication No. U.S.20040215345A1 entitled “Rotating/Non-Rotating Tibia Base Plate/Insert System.”